Why? Because the number on the scale isn't describing one thing, it's describing two things: fat mass and fat-free mass. One of them is fair game for burning, and the other is something you really want to hold on to! So the question we should be asking is: "How do I lose fat while keeping, or increasing, fat-free mass?"
Your fat-free mass includes your muscles, organs, bones, and connective tissue. It also includes water weight. In other words, this is what would be left if you removed every single fat cell from your body.
Muscle mass is a major component of your fat-free mass, and it should weigh more than your fat mass. Furthermore, muscle mass has a huge positive impact on your metabolism or "metabolic rate" which is how many calories your body burns for energy. The more muscle mass you have, the more calories you burn, even when you're not exercising.
However, muscle is also the physical engine that powers you through activity, both in the gym and in life. It also helps to support and strengthen your joints, helping to improve balance and reduce the risk of injury. Holding onto it should be a high priority, especially when dieting. And no, not just for bodybuilders!
But let's talk about fat. It gets a bad rap, but your body needs it, too! There is an essential amount that each body needs to be healthy. That amount will vary between body type, age, sex, physical activity level, and fitness goal. For the general population, the levels accepted as "healthy" are 21-32 percent for women and 8-19 percent for men. That's a big range!
To be clear, you can be higher than the "healthy" range and still be healthy, or be lower than it and be unhealthy. But the range is a pretty good place to aim for. Spending most of your life at higher levels can put you at risk for weight-related health problems such as Type 2 diabetes, certain types of cancer, and cardiovascular disease. Lower percentages can be fine for brief periods of time, but they can be very difficult to maintain and aren't ideal for long-term health.